Nitric oxide (NO) and hydrogen sulfide (H2S) are gasotransmitters (gaseous signaling molecules) with many different functions in human biology, including specific roles in cardiovascular physiology. H2S is a colorless, pungent gas that has multiple functions in physiology, including regulation of neurological function, intestinal inflammation, ischemia reperfusion injury, vasomotor tone regulation, and angiogenesis. NO plays critical roles in regulating vasomotor tone, inflammation, platelet activation, ischemic reperfusion injury, neurological function, and ischemic vascular remodeling. Both of these gaseous mediators are synthesized through enzymatic and non-enzymatic pathways. NO production classically occurs in an enzymatic manner via nitric oxide synthase isoenzymes, with non-enzymatic reduction of nitrite/nitrate serving as an alternative pathway. H2S is produced by three principal enzymes (cystathionine γ-lyase, cystathionine β-synthase, and 3-mercaptopyruvate transferase), with non-enzymatic generation via glutathione and other sulfane sulfur redox pathways. The biological effects of NO and H2S occur predominately through small molecule interactions and post-translational protein thiol modifications.
A clear understanding of H2S bioavailability and metabolism has been impeded by controversies in its measurement. It is now well-established that non-analytical methods have produced anomalous measurements. The most widely used assay, the methylene blue method, lacks specificity and is subject to various sources of error in the measurement of bioavailable free H2S. Aside from problematic assay methods, there remains a paucity of information regarding plasma free H2S levels in normal human physiology and disease states, and regarding the relationship between hydrogen sulfide and nitric oxide bioavailability. The few papers that have explored this area have been hampered by the use of older assay methods and their associated limitations. Recent advances have produced accurate and reliable analytical approaches for measuring free bioavailable H2S, including the ability to distinguish H2S bioavailability from different tissues. Information has also been obtained regarding plasma free H2S levels in young healthy adults; however, there is no comparable information for patients with cardiovascular disease.
It has recently been found that H2S can be used as an alternative to NO in promoting vasodilation. H2S also plays a role in regulating atherogenesis at the cellular and molecular levels. In addition, H2S can affect endothelial nitric oxide synthase (eNOS) expression and function while increasing nitrite anion reduction to NO, thereby providing an alternate pathway to regulate NO bioavailability. However, no information currently exists regarding biochemical relationships between NO and H2S in clinical settings.
Coronary Artery Disease (CAD) and Peripheral Artery Disease (PAD) currently afflict millions of adults in the U.S. and worldwide. However, early, asymptomatic CAD and PAD individuals often remain undiagnosed. Only about a third of U.S. PAD patients receive recommended therapies for secondary prevention. There exists an unfilled need for easy-to-implement diagnostic tests for PAD and CAD.
Several studies suggest that H2S plays important roles in atherosclerosis pathogenesis, ischemic vascular remodeling, and tissue ischemia reperfusion injury. H2S has been shown to decrease oxidation of low-density lipoprotein (LDL) and to decrease the uptake of oxidized LDL by macrophages in antioxidant responses. H2S impairs the migration of monocytes into the subendothelial layer via reduced expression of intercellular adhesion molecule-1 and monocyte chemoattractant protein-1. H2S has also been found to inhibit foam cell formation and to inhibit vascular smooth muscle cell proliferation. H2S has been reported to reduce vascular calcification in a rat model via down regulation of alkaline phosphatase activity and osteopontin gene down regulation.
Kleinbongard P, et al., Plasma nitrite concentrations reflect the degree of endothelial dysfunction in humans. Free radical biology & medicine. 2006; 40:295-302 reported that plasma nitrite levels are inversely related to the number of cardiovascular risk factors, such that the greater the number of risk factors, the lower the plasma nitrite levels.
In experimental systems it has been reported that plasma free H2S and NO have an influence upon one another. C. Coletta et al., Hydrogen sulfide and nitric oxide are mutually dependent in the regulation of angiogenesis and endothelium-dependent vasorelaxation. Proceedings of the National Academy of Sciences of the United States of America. 2012; 109:9161-9166.
Differences in total hydrogen sulfide or nitric oxide bioavailability have been reported in studies using various detection methods that cannot detect specific biochemical forms of hydrogen sulfide or nitric oxide metabolites (e.g., the methylene blue method or sulfide-sensitive electrodes). Differences in hydrogen sulfide levels may occur in congestive heart failure, Kawasaki disease, Alzheimer's disease, vascular dementia, hypertension, and coronary heart disease. No prior study has suggested that plasma hydrogen sulfide or nitric oxide metabolites are indicative of peripheral artery disease or coronary artery disease. See, e.g.: Chen L, Ingrid S, Ding Y G, Liu Y, Qi J G, Tang C S, Du J B: Imbalance of endogenous homocysteine and hydrogen sulfide metabolic pathway in essential hypertensive children. Chinese medical journal 2007; 120:389-393; Heiss C, Lauer T, Dejam A, Kleinbongard P, Hamada S, Rassaf T, Matern S, Feelisch M, Kelm M: Plasma nitroso compounds are decreased in patients with endothelial dysfunction. Journal of the American College of Cardiology 2006; 47:573-579; Jain S K, Bull R, Rains J L, Bass P F, Levine S N, Reddy S, McVie R, Bocchini J A: Low levels of hydrogen sulfide in the blood of diabetes patients and streptozotocin-treated rats causes vascular inflammation? Antioxidants & redox signaling 2010; 12:1333-1337; Jain S K, Manna P, Micinski D, Lieblong B J, Kahlon G, Morehead L, Hoeldtke R, Bass P F, 3rd, Levine S N: In African American type 2 diabetic patients, is vitamin D deficiency associated with lower blood levels of hydrogen sulfide and cyclic adenosine monophosphate, and elevated oxidative stress? Antioxidants & redox signaling 2013; 18:1154-1158; Jiang H L, Wu H C, Li Z L, Geng B, Tang C S: [Changes of the new gaseous transmitter H2S in patients with coronary heart disease]. Di 1 jun yi da xue xue bao=Academic journal of the first medical college of PLA 2005; 25:951-954; Kleinbongard P, Dejam A, Lauer T, Jax T, Kerber S, Gharini P, Balzer J, Zotz R B, Scharf R E, Willers R, Schechter A N, Feelisch M, Kelm M: Plasma nitrite concentrations reflect the degree of endothelial dysfunction in humans. Free radical biology & medicine 2006; 40:295-302; Kovacic D, Glavnik N, Marinsek M, Zagozen P, Rovan K, Goslar T, Mars T, Podbregar M: Total plasma sulfide in congestive heart failure. Journal of cardiac failure 2012; 18:541-548; Li X H, Zhang C Y, Wu J X, Zhang T: Changes in plasma hydrogen sulfide and nitric oxide levels and their clinical significance in children with Kawasaki disease. Chinese medical journal 2011; 124:3445-3449; Liu X Q, Liu X Q, Jiang P, Huang H, Yan Y: [Plasma levels of endogenous hydrogen sulfide and homocysteine in patients with Alzheimer's disease and vascular dementia and the significance thereof]. Zhonghua yi xue za zhi 2008; 88:2246-2249; and Sun N L, Xi Y, Yang S N, Ma Z, Tang C S: [Plasma hydrogen sulfide and homocysteine levels in hypertensive patients with different blood pressure levels and complications]. Zhonghua xin xue guan bing za zhi 2007; 35:1145-1148.
Numerous studies have tried to identify useful biomarkers for vascular diseases including PAD. Multiple indicators have been examined, including soluble adhesion molecules, inflammatory mediators, cytokines and other plasma proteins. However, the clinical utility of these markers as an indicator of vascular disease is still not clear and under active investigation. There is an unfilled need for improved biomarkers for cardiovascular disease.